<p>Microcracking and twinning are the two most popular mechanisms that operate in the brittle and semibrittle deformation regimes of rock. However, the complex temporal, spatial, and causal relationships between them has been discussed rarely. To reveal the possible interactions between them, a pure calcite marble was chosen to be triaxially compressed to introduce the two mechanisms. Optical observation afterward was conducted on the entire vertical cross-section perpendicular to the faulting surface. It suggests that in most circumstances, twinning occurs first and induces transgranular cracks by means of twin intersections, defects at the twin boundary, twin terminations, and grain yielding. Although sometimes twins appear later than microcracks or develop along grain boundary cracks, no conclusive evidence indicates them as microcrack-induced. The twins located in the faulting path evolve distinctively according to the orientations of the twin plane and the fault. As an indispensable procedure to the faulting of specimens, the fragmentation of thick twins in the shear zone involves at least four kinds of mechanisms to produce twin-breaking microcracks. In addition, discussions reveal the role of twin as an obstacle to the propagation of both microcrack and shear fracture. The statistical precedence of twinning over microcracking should responsible for the occurrence of large plastic deformation before the softening stage. Also, the anti-fracturing role of twinning partly explains the stable growth of fault in rock deformed in a semi-brittle regime.</p>